Transferred Touch: Sensory rewiring to improve prosthetics

Transferring a lost limb’s nerves to other areas of the body might one day permit an amputee to feel the heat of a coffee cup with an artificial hand. Scientists now report progress toward that goal. They’ve augmented a technique created several years ago to give patients control of prosthetic limbs.

HELPING HAND. Claudia Mitchell undergoes training to use her nerve-activated artificial limb. She is part of new research that may lead to prostheses that provide a sense of touch. Rehabilitation Inst. of Chicago

Todd A. Kuiken, a physician and an engineer at the Rehabilitation Institute of Chicago, and his colleagues developed a method called targeted reinnervation. They take nerves that originally went to an amputated limb and reroute what remains of them to muscles in the chest. In 2004, the researchers published results from the first patient to undergo the surgery, and they’ve since done the procedure on several more patients.

When the patient thinks about moving his arm, an electrical signal travels along the transferred nerves and activates the muscles where the nerves now end. An electrode resting on the skin picks up the muscles’ signal and sends it to the artificial arm, causing it to move.

“When the patient thinks ‘Close my hand,’ the [rewired] muscle acts as a biological amplifier of the nerve signal,” Kuiken says.

In new work, the first time the surgery was performed on a woman, Kuiken and his colleagues rerouted sensory as well as motor nerves during the reinnervation surgery. They transferred these nerves to the skin over the muscles that they co-opted.

In the Feb. 3 Lancet, the researchers report that after the surgery, the woman said that controlling her prosthetic arm felt natural.

The scientists also tested the patch of skin rewired with sensory nerves that originally extended to her lost hand. When the researchers applied pressure to different places on this skin, the patient reported tingling sensations that she associated with specific fingers or the palm of her lost hand. She also had sensations of temperatures and vibrations in her missing limb when the corresponding stimuli were applied to the rewired skin.

“The fact that they were able to provide this degree of use [with the prosthesis] and the potential for sensory control is really quite wonderful,” comments Gregory A. Clark, a bioengineer at the University of Utah in Salt Lake City.

Targeted reinnervation “is a very novel approach,” says Dawn M. Taylor, a biomedical engineer at Case Western Reserve University in Cleveland. The next task, she says, is to send information from the artificial limb to the sensory pathways.

Kuiken says that his group plans to collaborate with other research teams to develop prostheses with such sensory capabilities. For example, the researchers might place sensors on the digits of an artificial limb. Those sensors would relay information on stimuli to a device that applies the appropriate pressure or temperature to the rewired skin, he says.

Aimee Cunningham is the biomedical writer. She has a master’s degree in science journalism from New York University.

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